(1) Field of the Invention
The present invention relates to a discharge device, and in particular to a technology for improving a non-uniform discharge occurring in the discharge device for generating a corona discharge, and miniaturizing the discharge device.
(2) Description of the Related Art
Image forming apparatuses of an intermediate transfer method form a color image by overlaying toner images of, for example, colors Y (yellow), M (magenta), C (cyan) and K (black) on an intermediate transfer belt. The toner image carried by the intermediate transfer belt is transferred electrostatically onto a recording sheet by a second transfer roller, and then thermally fixed onto the recording sheet by a fixing device.
To transport the recording sheet carrying the toner image to the fixing device, the recording sheet needs to be separated from the intermediate transfer belt. However, when the recording sheet is stuck to the intermediate transfer belt by the electrostatic adsorption force or the like, the recording sheet might not be separated.
In view of the above problem, a separation claw, for example, may be used to separate the recording sheet from the intermediate transfer belt. However, since the separation claw contacts a surface of the recording sheet on which an image has been formed, the separation claw may scrape off unfixed toner, thereby causing an image noise to occur.
As another attempt, the curvature of the second transfer roller, on which the intermediate transfer belt is suspended, may be increased so that the recording sheet can be separated from the intermediate transfer belt by the rigidity (elastic restoring force) of the recording sheet itself. However, there is a limit to increasing the curvature of the second transfer roller.
In view of these circumstances, a method has been proposed. According to this method, the electrostatic adsorption force, which causes the recording sheet to be stuck to the intermediate transfer belt, is weakened by neutralizing the recording sheet by using the corona discharge (for example, see Japanese Patent Application Publication No. 09-114194).
However, the method of neutralizing the recording sheet does not necessarily separate the recording sheet from the intermediate transfer belt with absolute certainty. In particular, when the recording sheet is a sheet of thin paper, separating the recording sheet becomes further difficult since a sheet of thin paper is more deformable than a sheet of regular paper. This difficulty in separating the recording sheet is considered to be attributable to non-uniform neutralization of the recording sheet which is attributable to non-uniform discharge of the corona discharge.
With regard to the problem, one might consider as the countermeasure, for example, use of the scorotron structure or application of an alternating current (AC) voltage. However, with the scorotron structure, increase in the device size and cost cannot be avoided. Also, the application of an AC voltage brings a new problem that an AC noise may occur.
Furthermore, since there is no room around the second transfer roller, the corona discharge device needs to be miniaturized. The miniaturization of the corona discharge device is also a demand by a charging device which charges the photosensitive drum by the corona discharge.